Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging
Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging
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电荷对比成像揭示富钾碱长石的隐秘微观结构和地质历史
DOI：
10.1007/s00410-011-0710-8
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发表时间：
2011
影响因子：
3.5
通讯作者：
Flude S
中科院分区：
文献类型：
--
作者：
Flude S
扫描电子显微镜中的电荷衬度成像可以为碱长石显微结构的规模和组成提供新的见解，这些信息有助于解释其地质历史和氩同位素热年代学分析的结果。该技术的有效性已被证明使用富钾碱菱镁矿从达特穆尔花岗岩（英国）。这些长石含有应变控制的层状隐晶和微条纹长石，它们被无应变的氘代微条纹长石横切。两代微条纹长石的成分钠长石和正长石丰富的阶段，可以很容易地区分原子序数衬度成像。电荷对比结果还表明，亚微米大小的钠长石“血小板”是常见的粗出溶层片和发生在一起，使隐条纹长石。此外，电荷衬度成像显示，富含正长石的长石是两个相的共生体，一个是具有均匀对比度的无特征的，另一个是作为具有{110}冰长石习性的横切脉和颗粒出现的。透射电子显微镜观察表明，无特征的长石是花呢正长石，而静脉和自形颗粒组成的不规则微斜长石，已形成的正长石的“unzipping”在后成或热液蚀变。电荷衬度成像的结果是特别重要的，在证明氘条纹长石是更丰富的碱性长石比将从调查中得出结论，使用传统的显微镜技术。如此大量的替代产物的意外存在对于理解地壳岩石的起源和地质历史以及在地质和热年代学中使用碱性长石具有重要意义。虽然生成对比度的斜晶石的精确性质仍然不清楚，电荷对比度图像和相应的阴极射线发光图像的氘微条纹长石之间的相似性表明，微量元素化学和可能也弹性应变晶体内发挥了重要作用。
Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show that sub-micrometre-sized albite ‘platelets’ are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases, one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by ‘unzipping’ during deuteric or hydrothermal alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role.
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